In Alien Vs Predator a facehugger got onto a Predator, at the end of the film you see it bursting out of the dead Predator. In Alien Vs Predator (Requiem) which is the sequel, it's the main threat I think.

He is The Master-the creature occasionally referenced on Black Mountain Radio. Anyway, once you've saved your vault from dehydration by finding a water chip, you have to "stop the Super Mutants from attacking Vault 13". You do that by killing The Master. Refer to qpalz for more info.

Richard Grey, aka the Master, who was exploring Mariposa military base with Harold when both were exposed to Forced Evolutionary Virus (FEV), which is used to create super mutants. Harold left Richard for dead, and Richard mutated into that. He is the antagonist of Fallout. He believed that Super Mutants were the superior race and would lead the way into the future and dipped people in FEV to mutate them. He was killed when the Vault dweller blew up the Cathedral. :)